Cleaning contaminants from metal, machinery, precision parts, and textiles (dry cleaning) using hydrocarbon and halogenated solvents has been practiced for many years. Traditional dry cleaning machines operate typically as follows: a soiled garment is placed into a cylindrical "basket" inside a cleaning chamber which is then sealed. A non-polar hydrocarbon solvent is pumped into the chamber. The garment and solvent are mixed together by rotating the basket for the purpose of dissolving the soils and stains from the garment into the solvent, while the solvent is continuously filtered and recirculated in the chamber. After the cleaning cycle, most of the solvent is removed, filtered, and reused.
Recently the environmental, health, and cost risks associated with this practice has become obvious. Carbon dioxide holds potential advantages among other non-polar solvents for this type of cleaning. It avoids many of the environmental, health, hazard, and cost problems associated with more common solvents.
Liquid/supercritical fluid carbon dioxide has been suggested as an alternative to halocarbon solvents in removing organic and inorganic contaminants from the surfaces of metal parts and in cleaning fabrics. For example, NASA Technical Brief MFA-29611 entitled "Cleaning With Supercritical CO.sub.2 " (March 1979) discusses removal of oil and carbon tetrachloride residues from metal. In addition, Maffei, U.S. Pat. No. 4,012,194, issued Mar. 15, 1977, describes a dry cleaning system in which chilled liquid carbon dioxide is used to extract soils adhered to garments.
Such methods suggested for cleaning fabrics with a dense gas such as carbon dioxide have tended to be restricted in usefulness because they have been based on standard extraction processes where "clean" dense gas is pumped into a chamber containing the substrate and "dirty" dense gas is drained. This dilution process severely restricts the cleaning efficiency, which is needed for quick processing.
Another problem with attempts to use carbon dioxide in cleaning is the fact that the solvent power of dense carbon dioxide is not high compared to ordinary liquid solvents. Thus, there have been attempts to overcome this solvent limitation.
German Patent Application 3904514, published Aug. 23, 1990, describes a process in which supercritical fluid or fluid mixture, which includes polar cleaning promoters and surfactants, may be practiced for the cleaning or washing of clothing and textiles.
PCT/US89/04674, published Jun. 14, 1990, describes a process for removing two or more contaminants by contacting the contaminated substrate with a dense phase gas where the phase is then shifted between the liquid state and the supercritical state by varying the temperature. The phase shifting is said to provide removal of a variety of contaminants without the necessity of utilizing different solvents.
However, the problems of relatively slow processing, limited solvent power, and redeposition have seriously hindered the usefulness of carbon dioxide cleaning methods.
Another particularly serious obstacle to commercial acceptability of dense gas cleaning is the fact that when certain solid materials, such as polyester buttons on fabrics or polymer parts, are removed from a dense gas treatment they are liable to shatter or to be severely misshapened. This problem of surface blistering and cracking for buttons or other solids has prevented the commercial utilization of carbon dioxide cleaning for consumer clothing and electronic parts.